1. Field of the Invention
The present invention relates generally to a braking system for motorized vehicles having air brakes. More particularly the invention relates to electronically measuring air pressure and pushrod travel and determining from these two measurements whether the brakes are adjusted.
2. Description of the Related Art
Large trucks and tractor-trailers are becoming increasingly more common on our roadways. As a result, the safe operation of these vehicle has become a matter of increased concern. Of the accidents which are caused by mechanical failure, the greatest problem is improperly adjusted brakes.
Trucks and tractor-trailers operate on an air brake system. In an emergency situation the brakes must be properly adjusted. Most states have laws requiring trucks to demonstrate that their brakes are properly adjusted at an application pressure of up to 100 pounds of pressure per square inch (p.s.i.).
Referring to FIG. 1, truck brakes operate when an air chamber having air pressure applied to it pushes out a pushrod 14 which presses against a slack adjuster 22. The slack adjuster turns the camshaft 24 which causes the brake shoes 33-34 of FIG. 3 to be pushed against the brake drum 35, braking the vehicle. The pushrod 14 is extended from the air chamber 10 at low pressure until the brake shoes contact the brake drum surface. The pressure builds as the brake shoes are compressed against the brake drum. The slack adjuster, cam shaft and brake shoes have a certain amount of give to them. The process of taking up the give, and compressing the brake shoes against the drum is the windup.
The pushrod 14 has a maximum extension of about three inches. As the brake shoes wear down the amount of travel in the pushrod before the brake shoes contact the drum increases. Eventually it will reach a point where after the brake shoes have contacted the drum there is not enough extension left in the pushrod 14 to windup the brake up to the required braking force. Therefore, if a truck needs to brake in an emergency situation it is unable to do so because its brakes are so far out of adjustment the required braking pressure cannot be reached.
The term adjustment refers to the distance between brake drum and brake shoes. If the shoes are as close as they can be to the drum without contacting it the brakes are properly adjusted. As the brake shoes wear and the gap between them and the drum increases the brakes fall out of adjustment. Optimally, pushrod extension will cause immediate contact between the brake shoes and the drum. The remaining extension should then increase the pressure at which the shoes are compressed against the drum. As the brakes fall out of adjustment, some of the pushrod extension is used just to make contact between the shoes and the drum.
The prior art has come out with two devices which measure when the pushrod is extended too far out. The problem with these two devices, as will be seen, is that they measure the position of the pushrod which, in most instances, would only be reached in an emergency situation. While this measurement may be used to trigger a "brakes out of adjustment" warning signal, such an occurrence would be too late.
Referring to FIG. 1, one prior art system includes a sensor 42 which is set at a specified distance from the air chamber 10 along the pushrod's path of extension. A magnetic strip 40 is placed on the pushrod 14. When the pushrod 14 extends a specified distance, for example two inches, the magnetic strip 40 is positioned directly over the sensor 42 activating the sensor. The sensor unit sends a signal which could be displayed on the truck dashboard to flag the driver.
The second prior art method operates on the same principle. It has a built in indicator, set at two inches which clips out and is exposed when the pushrod reaches that length. The indicator is an orange band on the pushrod. During operation the orange band is not visible (it is inside the pushrod housing chamber). When the stroke reaches two inches the pushrod protrudes enough to expose the orange band which clips out and remains exposed.
Under normal braking conditions a truck uses about 30 or 40 p.s.i. of air pressure to stop. If the pushrod extends to 1.75 inches and is continually applying 30 or 40 p.s.i. neither of the above indicators will issue a "brakes out of adjustment" warning. In that case when an emergency stop is attempted and the 100 p.s.i. pressure is applied, the pushrod only has an inch or so to windup to the required braking force. Chances are significant that it will be unable to do this, prohibiting the truck from braking safely. Additionally, although the indicators would have been activated as the pushrod went through the two inch mark it would have been too late because the truck would have already been in an emergency situation.
This is, perhaps, clearer when reference is made to FIG. 6. Line C, which represent out of adjustment, travels to the right at low pressure initially. This is representative of the movement of the pushrod as it moves the brake shoes into contact with the brake drum. Once contact is made, at point C1, the pressure builds as the brake shoes are compressed against the drum with increasing force. Unfortunately, the pushrod had already extended too far before contact was made. Note specifically that line C reaches its full three inch extent before it ever reaches 100 p.s.i. Therefore, the brake will never be able to reach the required braking force, to stop in an emergency, and required by state law for inspection.
Conversely, at line A, the brakes are adjusted and pushrod extension causes the brake shoes to contact the brake drum almost instantaneously. The brake shoes can then be compressed against the brake drum with the requisite braking force within the two inches of travel of the pushrod.
Referring to line C again, if the prior art device is set at 2 or 21/4 inches and the truck has only had reason to brake under normal conditions (about 30 to 40 p.s.i.), a brake out of adjustment signal will never be given when, in fact, the brakes are already out of adjustment. When 100 p.s.i. is attempted it will fail. Although this failed attempt may trigger the prior art device by pushing the pushrod past 2 or 21/4 inches, it will be too late because the truck will not be able to obtain the necessary braking pressure.